<!DOCTYPE html>
<html>
<head>
<script src='http://code.jquery.com/jquery-latest.js'></script>
<style type='text/css'>
</style>
<script type='text/javascript'>
$(function (){
$('.qty__helper').click(function (event){
event.stopPropagation();
var o=$(this).siblings('.item-qty');
var val=parseInt(o.val());
val += $(this).hasClass("add") ? 1 : val == 0 ? 0 : -1;
o.val(val);
});
});
</script>
</head>
<body>
<div id="qty-spinner" class="qty">
<i data-helper="reduce" class="qty__helper reduce">-</i>
<input type="text" id="product_qty" class="item-qty" value="4">
<i data-helper="add" class="qty__helper add">+</i>
</div>
</body>
</html>
ksa, а у вас
молоко убежало в конце кода